The process of sizing a rotary dryer is one of precise engineering involving complex calculations and meticulous design. While the process is not well simplified into a few paragraphs, an explanation can be offered as to some of the concepts behind the process of rotary dryer sizing.
Many of the calculations behind sizing a dryer are based upon the characteristics of the material to be processed. The combination of characteristics like percent moisture, bulk density, specific heat, and heat transfer properties help predict how the material will behave in the dryer and subsequently, how best to address the needs of that material. These characteristics will not only help determine the sizing of the actual drum itself, but also the operational mechanics of the dryer.
Material Characteristics in That Impact Rotary Dryer Sizing
Inlet and Outlet Moisture
Because the primary job of a rotary dryer is to dry the material, the first material characteristic to look at is the percentage of moisture the material holds in its raw state, as well as the desired moisture percentage of the output, commonly referred to as the inlet and outlet moisture, respectively. The difference between actual and desired percent moisture sets the stage for what the rotary dryer will need to accomplish, and how hard it will have to work to get there.
Bulk Density
The bulk density of a material will also figure into the equation for sizing a rotary dryer. Typically, this is calculated in pounds per cubic foot in US units, or ks/m3 in metric units, and refers to the weight of a material per a specific volume. How much energy per cubic foot will in part determine how hard the dryer will have to work, which will in turn dictate bearings, gear train, trunnion wheels, and motor size.
Specific Heat & Heat Transfer Properties
The specific heat of a material also works into the sizing equation. Specific heat is defined as how much energy it takes to raise 1 gram of material 1 degree Celsius. In other words, it refers to how resistant a material is to heating. Put simply, the specific heat of a material will help to figure out how much energy is needed. Similarly, the heat transfer properties of the material will help decide whether a co-current or counter current dryer is needed.
Material Limitations That Play Into Sizing a Rotary Dryer
While not necessarily linked directly to the sizing, some material some materials will also present limitations that the dryer must be designed around. Most notably, this includes fragility, consistency, and end use.
Fragility
The dropping action used in a rotary dryer can be too aggressive for some materials that are more fragile. This might necessitate the use of a bald (flightless) section at the dryer inlet, which would allow the material a chance to harden a bit before being introduced to the dropping action.
Consistency
Materials that are sticky or sludge-like will also require special consideration during the design phase. This might include implementing a knocking system to reduce buildup, the use of a bald section, or even an adjustment to the flight design.
Additional precautions and modifications will need to be taken if the material is abrasive or corrosive as well.
End Use
The end use of a material can also put some limitations on the design. For example, some end product uses may demand absolute sterility, requiring special design modifications.
Conclusion
Though highly simplified, the concepts above offer a basic explanation for some of the material considerations behind sizing and designing a rotary dryer optimally suited for a given application. Sizing a rotary dryer often means finding the balance between how a material will behave in the dryer, the needed capacity, and the desired end product. FEECO has been engineering custom rotary dryers since 1951. To learn more about our custom rotary dryers, contact us today!